Many pathophysiological processes in the cardiovascular system such as thrombosis, vessel wall injury, and atherosclerosis occur in the presence of fluid shear forces. These shear forces have a direct mechanical effect on the vessel wall and can also indirectly affect wall properties by mediating the interactions of blood elements with the lumenal surface. The normal integrity of a vessel wall is maintained by a delicate monolayer of endothelial cells grown in monolayer culture. Morphological, cytoskeletal, and metabolic changes in the endothelial cells are being investigated as a function of fluid shear stresses on the cell surface. A parallel plate flow chamber was specially designed to accommodate Thermanox circular cover slips upon which human umbilical cord endothelial cells are cultured. The flow patterns and shear stress values within the chamber have been characterized using electrochemical shear measurements, flow visualization techniques, and laser doppler anemometry during steady flow. We have tested the chamber at shear stresses up to 20 dynes/cm2. The rate of development of cell orientation in the monolayer was observed as a function of shear stress from no flow to 16 dynes/cm2. Radio-immunoassays were developed to measure levels of von Willibrand factor, Factor VIII:R, and prostacyclin in the culture medium associated with release/production by the endothelial cells. The utility of these assays to follow the appearance of the substances as functions of time and shear stress intensity will be improved by increasing the proportion of monolayer area to culture medium volume. Alternatives to the parallel plate flow chamber with higher area-to-volume rations were examined. A cup-and-bob viscometric system was chosen for more detailed evaluation. One option being explored is to use the viscometer to shear suspension of cross-linked dextran microcarrier beads coated with the endothelial cell monolayers.